fix(core): stop requiring FileSystem to start a session

[K-Mistele]

Problem

startSession and resumeSession required FileSystem.FileSystem in every case. assembleSessionGraph ran yield* FileSystem.FileSystem unconditionally, and defineTool added FileSystem to every tool's dependencies. A session with a memory log and a pure tool (see fold-core/examples/AnthropicAgent.ts) still had to provide NodeFileSystem.layer. On a host without a filesystem, such as a Durable Object, the caller had to supply a fake one.

Change

The session no longer requires platform services. It picks FileSystem, Path and ChildProcessSpawner out of the caller's context, if present, and passes them to:

• tool init effects, at start and on switchModel
• the event log source's make
• the session service graph, which Effect AI's toolkit hands to tool handlers. Path and ChildProcessSpawner already reached handlers this way.

Descriptors that need disk keep working unchanged: coding tools declare platformToolDependencies, and jsonlEventLog and skillsFromDisk declare FileSystem. If one declares a service the caller left out, it dies with a missing-service defect when it asks for it, not at session start.

Other changes:

• Removed FileSystem from ToolHandlerServices, defineTool's default dependencies, liveToolRuntimeLayer, Toolset.handle and SessionProvisioningServices.
• Exported a PlatformServices type (previously the private PlatformToolServices).
• Removed NodeFileSystem.layer from the fold-core examples that don't use disk.

Tradeoff

A direct startSession caller that uses jsonlEventLog or the coding tools no longer gets a compile error if it forgets to provide FileSystem. It gets a defect at runtime instead, as Path and ChildProcessSpawner users already did. fold-agent's launchSession still requires FileSystem in its own signature, so the CLI path keeps the static check. Keeping the check everywhere would mean threading a generic R through FoldTool, agent and subagent definitions, and FoldSession. That is a much larger change.

The repo lint bans Effect.serviceOption, so the code reads the caller's context with Effect.context + Context.pick, the same way Effect AI's Toolkit.toHandlers captures handler context.

Tests

• New: a tool-calling session runs with no FileSystem provided.
• New: a handler that declares platformToolDependencies reads the FileSystem the caller provided.
• bun run typecheck, bun run test, bun run lint and bun run format:check all pass.

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